The art has long known of the practice of heating water only during flow demand periods, so to speak "continuous flow" heating, without the need for storage of heated water in tanks and the like. There is an evident efficiency in expending heating energy only at the time of demand, as contrasted with widely-used heated water reservoirs replenished with heat to maintain desired issuance temperature throughout extended non-demand periods.
One form of known continuous water heater apparatus is of the electrical resistance variety, wherein an electrical coil is arranged, with or without insulative jacket, in a housing through which cool, normal temperature water is directed on demand for heated water. Heretofore known apparatus of this type includes a control arrangement wherein alternating current power mains are switched into continuity with the electrical heating coil at zero-crossover voltage at the power mains, upon demand for heating, as sensed by a temperature sensor in the heater apparatus. Commercially known such apparatus and system is disclosed in published brochures of Thermar Corporation, Tankless Heater Division, Thermar Center, Trumbull, Connecticut 06611-0398, which are filed herewith in applicant's statement pursuant to 37 CFR 1.97 and 1.98.
Heretofore known electrical resistance heating apparatus and systems, such as the Thermar prior apparatus and other systems shown in the patents listed in the noted statement filed herein, have deficiencies in several respects, from applicant's viewpoint. Thus, such apparatus and systems previously known employ an electrical resistance heating coil which, while supported by a replacable cartridge and thereby readily replacable, experiences a relatively high rate of replacement due to burnout, attendant upon the course thereof under stress, i.e., that portion which is convoluted between the excitation power mains. More particularly, the coil is arranged in U-configuration, with its ends connected to electrodes at a common end face of the heater apparatus housing. The bottom of the U-configuration is in such return convolution and can be subjected to increased heating over the remnant unstressed courses of the coil and be more probable to burnout. Further, the radiant energy emitted by the coil has a tendency to becloud and confuse accurate temperature sensing of actual medium temperature. Additionally, switching means responsive to the temperature sensing is mounted on the apparatus housing and need be selected to have operating specifications avoiding the tendency to overheat based on the elevated temperatures of the heated medium. Need for improvement in these several respects was seen as essential by applicant, such that the industry could realize the benefits and economies of continuous flow heating apparatus and systems, above alluded to, over the constant heating demand, reservoir type, apparatus and systems presently in widespread use.